Mobile devices are typically hand-held devices that usually include a limited battery power source and a non-volatile memory for storing data when the mobile device is powered down. The power consumption needed to program/erase such non-volatile memory is a significant factor in how long a mobile device is able to operate on a given battery charge.
A radio frequency identification (RFID) system represents one type of mobile device that identifies unique items using an interrogator and an RFID device (sometimes referred to as a “tag”). Typically, the interrogator communicates with the RFID device that is attached to an item. The interrogator, also known as a reader, communicates with the RFID device through radio frequency signals, and passes the information read from the RFID device in digital form to a computer system. The RFID device is typically a microchip that stores the digital information using non-volatile memory cells. The microchip is attached to an antenna for communication with the interrogator. The RFID device includes a unique serial number and may include other information, such as a customer account number.
RFID devices can be active, passive, or semi-passive. Active RFID devices include a power source that powers the microchip's circuitry and transmits a signal to the interrogator. Passive RFID devices do not include a power source, and draw all the power required for the circuitry and the transmission of information from the radio frequency electromagnetic field generated by the interrogator. Semi-passive tags are similar to active tags, but their power source is only used to run the microchip's circuitry, not to communicate with the interrogator.
All power supplied to passive RFID devices is provided by the RF signal transmitted from the interrogator, and therefore must exhibit low power consumption of the embedded non-volatile memory in all operational states: programming, erasing, and readout. The currents utilized to facilitate the operational states of the non-volatile memory should typically not exceed 100 nA per cell (especially during flash erase operations), while the program/erase times must relatively short (less than several milliseconds). An additional requirement is that read-out operations must be performed at low voltages (i.e., at the level of 1V or below), because otherwise power-consuming additional charge pumps must be included in the RFID chip design. Moreover, the passive RFID device must be inexpensive to manufacture (i.e., not requiring additional masks or process steps in addition to the core CMOS process flow).
There are several conventional RFID device memory cell types.
A first type of RFID memory cell type uses the CMOS inverter principle for readout and is programmed by band-to-band tunneling (BBT) of electrons and erased by BBT holes. This first RFID memory cell type is disclosed, for example, in U.S. Pat. No. 6,788,576 (Complementary non-volatile memory cell; Yakov Roizin, 2004), which is incorporated herein by reference in its entirety. An advantage of this cell is that it requires very low read-out currents due to utilization of the CMOS inverter read-out principle. In addition, much lower program-erase currents (of the order of 300-500 nA, programming time approximately 100 μs, erase time approximately 20 ms) compared with memories employing channel-hot-electron (CHE) for programming. CHE programming requires at least tens of microAmps (see, e.g., U.S. Pat. No. 5,619,942, Stewart et al. , 1997)). Another advantage of BBT electron and hole injection mechanisms is their efficiency. In particular, one can strongly decrease the transfer point Vm of the FG CMOS inverter. A strongly negative Vm can be achieved using the voltages of the core CMOS circuit (below 5V), thus allowing sufficient erase margins with low Vread voltages.
Despite the mentioned advantages, the currents used in pure BBT programming/erasing single polycrystalline silicon memory devices are higher than required in modern passive RFID devices.
A solution that also employs a CMOS floating gate (FG) inverter, but uses Fowler-Nordheim (F-N) injection mechanism for charging and discharging the FG is disclosed in U.S. Pat. No. 5,272,368 (Turner, 1993). In the disclosed embodiments, special capacitors in the substrate are used: one is a tunneling capacitor through which the FG is charged and discharged, the second capacitor is a control capacitor. There are several limitations in the solution taught by Turner. First, it is difficult to erase a charge on the floating gate to negative values with small voltages (i.e., to extract electrons from the floating gate, or inject holes) because the value of the F-N current is exponentially dependent on the electric field in the tunnel oxide. The current significantly decreases during the erase operation. Attempts to use very high voltages (fields in the gate oxide exceeding 15 MV/cm) result in reliability limitations. Tunneling oxide charge-to-breakdown (Qbd) strongly decreases with the increase of the electrical field, thus limiting the device's endurance (i.e., the effective number of program/erase cycles; see, e.g., D. A. Buchanan “Scaling of gate dielectrics: Materials, Integration and Reliability, IBM Journal of R&D, v. 43, 1999, pp. 245-264). Also, special implants (diffusions) are disclosed in U.S. Pat. No. 5,272,368 to fabricate tunnel and control capacitors. These implants require additional masking steps if implemented in standard CMOS process flows. Moreover, when high voltages are employed for programming and erase (programming with plus 12V at control capacitor, tunnel capacitor terminal grounded, erase with 12V at tunnel capacitor, control capacitor terminal grounded, excess currents (usually of “gate induced drain leakage” (GIDL) origin) flow in the peripheral circuits.
What is needed is a non-volatile (floating gate) memory cell for mobile devices that avoids the problems associated with the prior art structures discussed above. In particular, what is needed is an electrically erasable/programmable CMOS logic memory cell and associated programming/erasing algorithm that functions at low voltage (i.e., below 5V).